There are several different systems already available or under development to reduce the levels of the more harmful constituents of exhaust gas. These include:

Selective Catalytic Reduction (SCR), which uses an ammonia solution injected into the exhaust to react with the NOx to produce harmless nitrogen and water vapour.

DeNox catalysts which remove NOx by adsorption and then break it down in the presence of a small quantity of diesel fuel particulate traps, which filter out the micron sized particulates from the exhaust. Regenerative traps burn the collected particulates during periods of higher exhaust temperatures, aided by the presence of an oxidising catalyst.

Any treatment system in the exhaust pipe reduces the apparent turbo efficiency by adding back pressure so turbocharger design and application will need to offset this. The other problems with these are their extra cost and the extra space taken up by the additions to the exhaust system.